dhenry:
I would put a small resistor (ideally 10mohm but higher is ok given that the current isn't too big) from the source to ground.You should see voltage pulses on this resistor (6mv or so) when the motor is running, and nothing when the motor is disconnected.
As the count resolution for a arduino analog input pin is 5 millivolts or so and who knows what the noise floor will be, this seems like a most undesirable unreliable measurement to use to determine if the motor is operating at normal current draw or not.
LeftyWhether you use a low-pass filter after that will depend on your particular solution.
You can also detect a stuck motor (connected but not running): you will see higher magnitude of the pulses.